Open-web rafters

Posted:
12/04/2012 03:00:00 AM EST

Tuesday December 4, 2012

Last week I wrote about an innovative foundation insulation material, Foamglas, that we used in our new house in Dummerston. This week I’ll talk about the open-web rafters we’re using to achieve a superinsulated roof.

First, a little background. To create highly insulated roofs there are several approaches:

When the insulation is installed in the attic floor (creating an unheated attic), it’s easy to obtain very high R-values inexpensively -- it’s cheap, that is, as long as you don’t count the cost of the lost living space by creating an unheated attic. Basically, you just dump in a lot of loose-fill cellulose or fiberglass on the attic floor, filling the joist cavity and more. I’ve heard of as much as two feet of cellulose insulation being installed in this manner, achieving about R-80. To make room for a lot of insulation at the roof eaves, it’s usually necessary to install "raised-heel" trusses for the roof framing (so that the insulation thickness at the edges is not significantly compromised.

If you want to insulate the sloped roof, creating living space -- as we are doing -- you can either install very thick rafters (14 inches or more) that can be filled with cavity-fill insulation, or you can provide more modest roof trusses or rafters and then add a layer of rigid insulation on top of the roof sheathing. An advantage of this approach is that the layer of rigid insulation controls the "thermal bridging" through the rafters or top chords of the roof trusses.

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To keep the insulation costs down and minimize our use of foam-plastic insulation, we opted for the former option -- putting all our insulation in the rafter cavities rather than installing a second layer of outboard insulation.

Finding deep-enough rafters

To achieve the 16-inch depth we wanted for insulation and an air space under the sheathing, we used open-web, parallel-chord trusses as the rafters. These trusses, typically used as joists, have diagonal bracing or "struts" and are made by in Quebec by Open Joist Triforce (www.openjoisttriforce.com).

Unlike most parallel-chord trusses, Tri-Force uses solid wood, rather than OSB, and finger-jointed glue joints rather than metal truss plates for attaching chords and webs. (You will be able to see photos in the blog version of this column on BuildingGreen.com.) Some experts are concerned about the long-term durability of OSB webs in more common I-joists and the metal fasteners in standard roof trusses.

The chords on Triforce joists are either 2x3s or 2x4s, and the diagonal struts are solid-wood 2x2s. Connections between 2x2 webs and shorts are achieved with precision-machined grooves and polyurethane adhesive. The wood is all northern, slow-grown spruce, rather than plantation-grown southern yellow pine or poplar. (Seeing the photos at BuildingGreen.com will make this more understandable.)

Triforce joists include a section of OSB at the ends so that the length can be adjusted. This permits manufacturing in standard lengths and keeps the costs down.

Providing a stem wall
and roof overhang

In our case, to expand the living area in the upstairs of our compact house, Eli Gould added "raised heels" to the roof trusses. The OSB tails on the Triforce rafters made this fairly straight-forward, though it certainly involved some additional labor. The design at the roof eaves also provides for nearly two feet of overhang -- a high priority in keeping moisture off the wall and away from windows and foundation.

Despite the extra work with the raised-heel and overhang, the rafters went up quickly. Eli’s crew worked all-day on the Saturday before Superstorm Sandy came through to get the roof up and sheathed with Huber’s Zip sheathing (with joints taped). They were able to keep everything remarkably dry.

Insulation options

We have not made a final decision about the type of insulation we will use for the roof. We are deciding between dense-pack cellulose and acrylic-stabilized, blown-in fiberglass (probably the Johns Manville Spider product). With 14 inches of insulation, the difference in weight between cellulose (at about three pounds per cubic foot) and Spider (1.8 pounds per cubic foot) is significant.

With either material, we believe that by stapling up mesh-fabric baffle on each rafter we will be able to fill each rafter cavity completely -- including all the corners where the diagonal struts intersect the chords. The small amount of acrylic adhesive in the JM Spider product may prove to be a significant benefit to us in fully sealing the cavities -- so we’re leaning in that direction.

The two materials provide similar insulation values: about R-4.1 to 4.2 per inch for the JM Spider fiberglass and about R-3.7 per inch for dense-pack cellulose. With 14 inches of insulation, that would come to about R-58 with JM Spider, vs. R-52 with dense-pack cellulose.

From an environmental standpoint, cellulose has higher recycled content (about 80 percent recycled newspaper), though fiberglass insulation is now made using a significant amount of recycled glass (mostly from beverage containers). Johns Manville fiberglass is certified to have minimum 25% recycled glass content (with 80 percent of that recycled content being post-consumer).

Flame retardants are not required in the fiberglass, while borate and ammonium sulfate flame retardants are used in cellulose.

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